Illustrations

In-Depth From A.D.A.M. Background

Herpes simplex virus (HSV) commonly causes infections of the skin and mucous membranes. Sometimes it can cause more serious infections in other parts of the body. HSV is one of the most difficult viruses to control and has plagued mankind for thousands of years.

Herpes simplex is part of a group of other herpes viruses that include human herpes virus 8 (the cause of Kaposi's sarcoma) and herpes zoster (the virus responsible for shingles and chicken pox). There are more than 80 types of herpes viruses. They differ in many ways, but the viruses share certain characteristics, notably the word "herpes," which is derived from a Greek word meaning "to creep." This refers to the unique characteristic pattern of all herpes viruses to "creep along" local nerve pathways to the nerve clusters at the end, where they remain in an inactive state for some indeterminate time.

There are two forms of the herpes simplex virus:

Herpes simplex virus 1 (HSV-1)

Herpes simplex virus 2 (HSV-2)

These viruses are distinguished by different proteins on their surfaces. They can occur separately, or they can both infect the same individual. Until recently, the general rule was to assume that HSV-1 infections occur in the oral cavity (mouth) and are not sexually transmitted, while HSV-2 attacks the genital area and is sexually transmitted. It is now widely accepted, however, that either type can be found in either area and at other sites. In fact, HSV-1 is now responsible for up to half of all new cases of genital herpes in developed countries.

The Disease Process

For infection to occur, the following conditions must apply:

The herpes simplex virus passes moves through bodily fluids (saliva, semen, fluid in the female genital tract) or in fluid from herpes sores.

The virus must have direct access to the noninfected person through injuries in their skin or mucus membranes (such as in the mouth or genital area).

When herpes simplex virus enters the body, the infection process typically takes place as follows:

The virus enters vulnerable cells in the lower layers of skin tissue and tries to reproduce in the cell nuclei.

Even after it has entered the cells, the virus never causes symptoms in most cases.

However, if the virus destroys the host cells when it multiplies, inflammation and fluid-filled blisters or ulcers appear. Once the fluid is absorbed, scabs form, and the blisters disappear without scarring.

After the first time they multiply, the viral particles are carried from the skin through branches of nerve cells to clusters at the nerve-cell ends (the
dorsal root ganglia
).

Here, the virus lives in an inactive (
latent
) form. The virus does not multiply, but both the host cells and the virus survive.

At unpredictable times, the virus begins multiplying again. It then goes through a period called
shedding
. During those times, the virus can be passed into bodily fluids and infect other people. Unfortunately, a third to half of the times shedding occurs without any symptoms at all.

Eventually, the symptoms return in most cases, causing a new outbreak of blisters and sores.

Transmission

To infect people, the herpes simplex viruses (both HSV-1 and HSV-2) must get into the body through broken skin or a mucous membrane, such as inside the mouth or on the genital area. Each virus can be carried in bodily fluids (saliva, semen, fluid in the female genital tract) or in fluid from herpes sores. The risk for infection is highest with direct contact of blisters or sores during an outbreak.

Once the virus has contact with the mucous membranes or skin wounds, it begins to replicate. The virus is then transported within nerve cells to their roots where it remains inactive (
latent
) for some period of time. During inactive periods, the virus cannot be transmitted to another person. However, at some point, it often begins to multiply again without causing symptoms (called
asymptomatic shedding
). During shedding, the virus can infect other people through exchange of bodily fluids.

Sometimes, infected people can transmit the virus and infect other parts of their own bodies (most often the hands, thighs, or buttocks). This process, known as autoinoculation, is uncommon, since people generally develop antibodies that protect against this problem.

Transmission of Oral Herpes
. Oral herpes is usually caused by HSV-1. HSV-1 is the most prevalent form of herpes simplex virus, and infection is most likely to occur during preschool years. Oral herpes is easily spread by direct exposure to saliva or even from droplets in breath. Skin contact with infected areas is enough to spread it. Transmission most often occurs through close personal contact, such as kissing. In addition, because herpes simplex virus 1 can be passed in saliva, people should also avoid sharing toothbrushes or eating utensils with an infected person.

Transmission of Genital Herpes
. Genital herpes is most often transmitted through sexual activity, and people with multiple sexual partners are at high risk. The virus, however, can also enter through the anus, skin, and other areas.

People with active symptoms of genital herpes are at very high risk for transmitting the infection. Unfortunately, evidence suggests about a third of all herpes simplex virus 2 (HSV-2) infections occur when the virus is shedding but producing no symptoms. Most people either have no symptoms or don't recognize them when they appear.

In the past, genital herpes was mostly caused by HSV-2, but HSV-1 genital infection is increasing. This may be due to the increase in oral sex activity among young adults.

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